Angiopoietin-2 facilitates vascular endothelial growth factor-induced angiogenesis in the mature mouse brain

A better understanding of angiogenic factors and their effects on cerebral angiogenesis is necessary for the development of effective therapeutic strategies for ischemic brain injury. Vascular endothelial growth factor (VEGF) has been shown to induce angiogenesis in the adult mouse brain. However, the function of angiopoietin-2 (Ang-2) in cerebral angiogenesis has not been clarified. The goal of this study was to identify the combined effects of VEGF and Ang-2 on cerebral angiogenesis and the blood-brain barrier (BBB).

Six groups of 6 adult male CD-1 mice underwent AdlacZ (viral vector control), AdVEGF, AdAng2, VEGF protein, VEGF protein plus AdAng2, or saline (negative control) injection. Microvessels were counted using lectin staining on tissue sections after 2 weeks of treatment. Matrix metalloproteinase-9 (MMP-9) activity was determined by zymography. The presence of zonula occludens-1 (ZO-1) protein was determined by Western blot and immunohistochemistry.

Mice treated with VEGF protein infusion plus AdAng-2 significantly increased microvessel counts relative to all other groups (P<0.05). The changes in MMP-9 activity paralleled the reduced ZO-1 expression in the VEGF plus Ang-2-treated group compared with the other 5 groups (P<0.05). Double-labeled immunostaining demonstrated that ZO-1-positive staining was significantly decreased on the microvessel wall in the VEGF plus Ang-2-treated group.

Our study demonstrates that the combination of VEGF and Ang-2 promotes more angiogenesis compared with VEGF alone. Furthermore, the combination of VEGF and Ang-2 may lead to BBB disruption because it increases MMP-9 activity and inhibits ZO-1 expression.